The project goal is to develop a photochemical method for investigating and modifying the mechanism of protein-based enzyme catalysis. The method will be demonstrated in the copper amine oxidase system. Catalytically-relevant topa quinone (TPQ) radical will be generated in protein which has been modified by replacing the redox active copper cofactor with an organic substrate linked through a hydrocarbon chain to a photochemical sensitizer on the outside of the enzyme. The influence of the active site metal and pH on the structure and reactivity of the TPQ radical will be studied using a combination of metal replacement experiments and chemical modification of the substrate. The method will be extended to perform photocatalysis of amine oxidation under anaerobic conditions.